A radar system transmits energy to the surrounding environment in which a specific target or class of targets is desired to be detected. If the environment contains unwanted radar returns such as weather clutter, sea clutter and land clutter, the desired target or targets are generally masked or obscured by these rather strong returns. To overcome this problem, signal processing can be employed to enhance the target-to-clutter power ratio. However, to employ signal processing a discriminant is required -- that the target contain a unique feature that the clutter does not contain. Historically, doppler filters have been employed for target-to-clutter enhancement. A moving target has a doppler frequency offset where nonmoving clutter can be rejected by a doppler filter. If, however, the target and clutter have the same velocity components, the velocity discriminant is lacking and the target-to-clutter cannot be enhanced by doppler processing.
The method and system as described herein employs a discriminant which is derived from the target's unique shape and radar cross section as a function of the polarization of the radar. The polarization of the transmitted waveform is rotated very fast at a constant angular rate. As described in detail hereinafter the polarization is rotated more than 360.degree. during the transmitted pulsed waveform. The received waveform from the target, if received with the same polarization that was transmitted will be modulated by the target. The unique modulation will be a function of the target's polarization behavior. Under the condition that the energy received has the same polarization as the transmitted polarization and both are rotating at a constant linear rate, the energy in the receiver will contain frequency components that were not transmitted. Thus, for some types of targets a receiver may be tuned to a frequency band that was not transmitted and target discrimination may be obtained.
The present invention provides features such as a novel radar system which enhances target-to-clutter visibility even for slow or nonmoving targets. Further, the method can be employed for target identification. A new principle is utilized for radiating an electromagnetic waveform where the polarization is rotated at a very fast rate heretofore unobtainable with conventional polarizers. In addition, there is presented a new principle for effectively receiving with a fast rotating polarization by employing signal processing techniques. There is also provided a signal processing technique for obtaining target-to-clutter enhancement while achieving a maximum target signal-to-receiver noise ratio. Still further, there can be performed the enhancement and/or identification with a single pulse. Finally, the aforementioned feature may be achieved with a radar system in which the signal path in the radar remains constant whereby other forms of signal processing can also be used simultaneously.
The present radar system permits detecting targets which are obscured by strong unwanted clutter returns. The principle herein does not require that the target be moving. Also the identification of target classes can be performed. The concepts can be employed with radar for broad applications including surveillance, target tracking, weapon guidance, and target identification in both ground-based and space-borne radars.